1. Field of the Invention
The invention relates to a buttonhole sewing machine for the production of buttonholes on a workpiece, comprising a needle mounted in an arm, which is reciprocatingly drivable in a Z direction for the production of stitches by means of a driving motor, and which is drivable by a jogging drive for the production of a zigzag seam by a motion of the needle relative to the workpiece, and which is drivable to pivot about an axis by means of a pivot drive; a hook bearing, which is disposed in a base plate, and which is drivable by a pivot drive to pivot synchronously and equiangularly relative to the needle about a pivot axis which extends in the Z direction; and a thread feeding mechanism in the path of a needle thread.
2. Background Art
In a buttonhole sewing machine of the generic type known from U.S. Pat. No. 1,372,473, which is designed for the production of eye type buttonholes, stitch forming takes place in a zigzagging sequence of stitches in a conventional and known manner by alternating single thread chain stitches and double thread chain stitches. In the case of a single thread chain stitch, no under-thread or hook thread is fed, whereas an under-thread or hook thread is fed in the case of a double thread chain stitch. This known buttonhole sewing machine is provided with a device for the control of the needle thread, in which, on a shaft that rotates at half the speed of the arm shaft a pair of disks, which co-rotate therewith, and a cam are disposed for the control of a thread clamp.
U.S. Pat. No. 4,590,879 teaches a thread feeding mechanism of a sewing machine, in which a cam disk is provided, which rotates at half the speed of the arm shaft and which, by two portions on its periphery that are remote from the axis of rotation and by two portions that are close to the axis of rotation, acts on the thread supplied to the needle in such a way that the thread is tensioned, i.e. it is pulled, or loosened, i.e. released.